Chiral Benzylic Carbocations: Low-Temperature NMR Studies and Theoretical Calculations

The α-chiral secondary and tertiary benzylic carbocations 19−30 were generated from the corresponding benzylic alcohols 1, 2, and 5−14 by treatment with FSO₃H or FSO₃H/SbF₅ in SO₂ClF as the solvent at −70 °C and characterized by one- and two-dimensional NMR spectroscopy. Coupling constants and NOESY measurements suggest a preferred conformation in which the α-hydrogen atom occupies the 1,3-allylic-strain position and the diastereotopic faces of the cations are differentiated by the alkyl substituent and a functional group (FG). The existence of this preferred conformation is further supported by calculations using a DFT method at the B3LYP/6-311+G** level. Quenching experiments with an arene nucleophile showed a preferential attack from the less shielded diastereotopic face delivering high diastereomeric ratios, supporting the hypothesis that these carbocations are involved as intermediates in previously studied S_N1 reactions. A strong shielding effect at the benzylic carbocationic center is observed for most of the secondary benzylic carbocations (derived from precursors 5−13) investigated, indicating a strong mesomeric distribution of the positive charge to the carbon atom in the para-position of the anisyl substituent. For α-halogen-substituted carbocations (5−7, 12), no neighboring halogen participation leading to halonium ion formation was observed.